Light-sensitive photographic material with increased sensitivity to light

ABSTRACT

1. A LIGHT SENSITIVE PHOTOGRAPHIC MATERIAL OF IMPROVED SPEED AND GRANURALITY ESSENTIALLY CONSISTING OF A SUPPORT AND IN SUPERPOSED RELATIONSHIP A FIRST SILVER HALIDE EMULSION OF SILVER BROMIDE OR SILVER BROMO-IODIDE CLOSER TO THE SUPPORT AND A SECOND SILVER HALIDE EMULSION OF SILVER BROMIDE OR SILVER BROMO-IODIDE SUPERPOSED OVER THE FIRST EMULSION WHEREIN THE IMPROVEMENT COMPRISES THE SECOND EMULSION IS IN A LAYER HAVING A THICKNESS OF AT LAST 2$ AND THE FIRST EMULSION IS IN A LAYER HAVING A THICKNESS OF BETWEEN 5 AND 15U AND IS GREATER IN THICKNESS, THE FIRST EMULSION HAS A SENSITIVITY ABOUT 0.1 TO 0.4 LOG 1.T UNITS LESS THAN THE SENSITIVITY OF THE SECOND EMULSION AS MEASURED BY THE POINTS ON THE CHARACTERISTIC CURVES WHICH   ARE ABOVE THE EXPOSURE REQUIRED TO PRODUCE VISIBLE DENSITY, AND WHICH IMPROVEMENT RESULTS IN THE FIRST EMULSION GIVING A DENSITY OF AT LEAST 1.5, THE SECOND EMULSION GIVIMG A DENSITY OF NOT OVER 0.9 AND THE SLOWER EMULSION HAVING THE THRESHOLD OF ITS CHARACTERISTIC CURVE COINCIDING IN LOG 1.T UNITS WITH THE SHOULDER OF THE CHARACTERISTIC CURVE OF THE FASTER EMULSION AND WHEREBY OVERALL SENSITIVITY IN LOG 1.T UNITS OF SAID LIGHT SENSITIVE MATERIAL IS GREATER THEN THE SENSITIVITY OF THE MORE SENSITIVE SECOND EMULSION MEASURED BY THE POINTS ON THE CHARATERISTIC CURVE ABOVE THE EXPOSURE REQUIRED TO PRODUCE VISHIBLE DENSITY.

Nov. 5, 1974 E. HELLMIG E AL 3,846,135

LIGHT-SENSITIVE PHOTOGRAPHIC MATERIAL WITH INCREASED SENSITIVITY TOLIGHT Filed April 8. 1971 INVENTORS:

EHRHARB HELLMIG, FRANZ EvICiJL, AUGUST RANDOLPH, WILHELM SALECK.

United States Patent ()flice 3,846,135 Patented Nov. 5, 1974 1m. 01.G03c 1/02, 1/08, N46

US. Cl. 96-68 3 Claims ABSTRACT OF THE DISCLOSURE Synergistic increasein light sensitivity of combination of two superposed silver halideemulsion layers is obtained where lower layer is less sensitive thanupper layer, has a density of at least 1.5 as against a maximum densityof 0.9 for the upper layer, the characteristic curve of the upper layeris no steeper than that of the lower layer, the lower layer is about 5to 15/L thick and thicker than the upper layer. Both emulsions arepreferably iodobromide emulsions, and can be panchromaticallysensitized.

This application is a continuation-in-part of our copending US.Application Ser. No. 634,507 filed Apr. 28, 1967, entitled LightSensitive Photographic Material With Increased Sensitivity to Light, nowabandoned.

The invention relates to a photographic black-white material withincreased sensitivity.

It is well known that the greater the sensitivity of a light-sensitivephotographic silver halide emulsion, the coarser will be the silvergrain after development of the exposed material. The production ofphotographic layers of high light sensitivity is therefore limited bythe coarseness of the grain. This applies equally to black and whitefilms and to color photographic elements, because of the degree ofgranularity of the silver halide grain of the emulsion used.

Since the graininess of the color image interferes particularly stronglywith the overall impression of the image, epecially on projection, andsince photographic practice demands highly sensitive and thereforecoarse-grained emulsions, there has been a search for means forincreasing the sensitivity of color photographic layers withoutconcomitant increases in grain. It is known to apply over a fine-grainedsilver halide emulsion of medium sensitivity a second silver halideemulsion of higher sensitivity and hence also coarser grain, which hasthe same silver content but a lower color coupler content so that aftercolor development, the second layer assumes only a limited maximum colordensity, e.g., of 0.20 to 0.60, even at maximum exposure. Although thedesired increased sensitivity is thus provided by the second layer, theinfluence of the coarser grain on the total grain of the double layerremains relatively slight.

It is among the objects of the present invention to produce a black andwhite photographic material with increased sesitivity withoutconcomitant increases in granularity of the grain.

We now have found that the sensitivity of black and white photographicmaterials can be considerably increased by the use of double silverhalide emulsion layers if certain conditions are observed as regards theproportions of the silver halide contents in the two silver halideemulsion layers and the density of the more sensitive layer. In thearrangement of layers of the invention, the more sensitive silver halideemulsion layer, which is arranged on the top, contains per unit ofsurface at most 60% of silver halide based on the silver halide contentof the less sensitive lower silver halide emulsion layer. The silverhalide content of the more sensitive layer should be that suflicient toprovide a maximum density of not more than 0.90 preferably between 0.50and 0.70, while that of the lower provides a density of at least 1.5with a straight characteristic.

The difference of sensitivity between the more sensitive silver halideemulsion layer and the less sensitive silver halide emulsion layer ispreferably 0.1 to 0.4 log 1.)? units.

With the threshold of the characteristic curve of the less sensitiveemulsion coinciding with the shoulder of the characteristic curve of themore sensitive emulsion flatter, very good results are obtained. Thesensitivity curves are those of a density-log exposure graph forphotographic emulsions. The coordinates on which such curves arecustomarily plotted are the density which is indicated along theordinate. The threshold is the log I.t value on the abscissa at whichthe plotted photosensitivity emulsion becomes responsive to light. Theshoulder is the log I.t unit value where the emulsion approachessaturation.

The accompanying Figure shows the characteristic curves of theindividual emulsions and the curve which results from the combination ofthe emulsions according to this invention.

According to a preferred embodiment of the invention, the more sensitivesilver halide emulsion layer is thinner than the less sensitive layerunderneath it.

The thickness of the more sensitive silver halide emulsion layer liesbetween 2 and 7,u, while the less sensitive layer has a thickness ofbetween 5 and 15,u.

It is unexpected that in the black and white photographic elementaccording to the invention the sensitivity of the double layered elementis not equal to the sensitivity of the more sensitive top layer butconsiderably higher. Thus, when the double layer element of theinvention is used a synergistic effect appears and a very highlight-sensitive photographic material results.

The usual silver halide emulsions can be used for production of thedouble layered element of the invention; it is preferred, however, touse silver bromide gelatin emulsions that have a minor silver iodidecontent of up to about 10 mols percent. If required, however, silverchloride emulsions may also be used, if desired in admixture with theabove mentioned silver halides.

The light-sensitive emulsion to be used according to the invention maycontain the usual additives such as chemical or optical sensitizers,stabilizers, wetting agents, hardeners, etc.

Photographic materials prepared according to the invention can be usedin all fields of photography. They are of special importance forreproduction work where there is a demand for panchromatic half-tonefilms of maximum sensitivity which are commonly used for the productionof color separation records. The use of narrow band color separationfilters which absorb a considerable amount of light as well as thecombination of the original with a masking image and in many cases thenecessity of producing an enlargement of the original or a portion of itdiminish the amount of light available to such an extent that theexposure time for the separation records is in many cases increased toan unacceptable degree.

EXAMPLE 1 A panchromatically sensitized silver halide gelatin emulsioncontaining per kg. g. of gelatin, 0.55 mol silver halide (silver bromideplus 4% silver iodide) which has been after-ripened as described byGlafkides Photographic Chemistry Vol. 1, 2nd Edition, page 319, in thepresence of 15 mg. of thiosulfate and 1.6 mg. of gold in the form ofammonium aurothiocyanate per liter and which further contains 50 ml. of1% methanolic solution of -methyl-7-oxy-1,3,4-triazaindolizine, ml. of a10% aqueous solution of formaldehyde and ml. of a 7.5% aqueous solutionof saponine is applied onto a support of polyethylene glycolterephthalate.

Onto that layer is applied a second layer of a panchromaticallysensitized silver bromide emulsion containing 2 mol percent of silveriodide. The emulsion is an ammonia emulsion containing the additivesreferred to above and is further chemically sensitized by addition of apentaerythritol phosphoric acid ester as described in British Pat. No.1,045,184.

Onto that second silver halide emulsion layer is applied an antistaticgelatin layer (0.5,u in thickness) which protects the silver halidelayers against mechanical damage The thickness of the lower layer is 7.6its silver content in the form of silver halide is 4.50 g./m. the upperlayer has a thickness of 4.7,u and a sliver content in the form ofsilver halide 2.43 g./m.

The finished material is exposed behind a test wedge in a sensitometercustomarily employed in the art. The sensitivity of the more sensitivetop layer, measured at a density of 0.10 over fogging is only 0.25 log1.1 units higher than that of the less sensitive layer. The double layerelement, however, has a sensitivity of 0.46 log I.t units above that ofthe less sensitive layer. An additional increase in sensitivity of 0.21has thus been obtained by the greater than additive effect of theinventive double layer element. This increase amount to the factor 1.62or 62%. Compared with the less sensitive lower layer, the sensitivityincrease of 0.46 log units amounts to 188% (factor 2.88) without anycoarsening of the silver grain.

The results are set forth in the Figure. The Figure shows curves thatare based on the description of Example 1:

1. The emulsion B has sensitivity of about 0.1 to 0.4 log 1.: units lessthan the sensitivity of the upper emulsion.

2. The shoulder of the curve of the upper emulsion A coincides with thethreshold of the curve of the emulsion B.

3. The curves of emulsions A and B have substantially the same shape.

4. The upper emulsion A has a maximum density of not more than 0.9 andthe emulsion B has a maximum density of at least 1.5.

5. The sensitivity of the combination of the emulsions is substantiallyhigher in sensitivity than the emulsions A and B and the differencebetween the sensitivity of the combination and emulsion B is greaterthan the difference between the sensitivities of emulsions A and B. Thesensitivity of the combined emulsions is substantially different fromthe sensitivity of the upper emulsion, which is the one with the highersensitivity.

EXAMPLE 2 A maximum sensitivity emulsion containing 5 mols percent ofsilver iodide is substituted for the less sensitive layer described inExample 1. The same additives as listed in Example 1 are added to thefinal after-ripened emulsion. Thickness of lower layer: 7.6,u/upperlayer: 4.6 Silver content of lower layer: 4.5g./m.-/upper layer: 1.77g./m. in the form of silver halide Sensitivity upper/lower layer=0.1 logI .t

0.1 over Sensitivity total/lower layer=0.3 log I .t foggmg' This exampleshows that the synergistic effect has led to a sensitivity increase of0.2 log. 1.! units, in other Words to a 58% higher sensitivity.

It is thus possible to produce light-sensitive photographic materialswith an unexpectedly high sensitivity without concomitant coarsening ofthe silver halide grain.

If this sensitivity increase in panchromatic films is to be obtained forlight of all three-thirds of the visible spectrum, that is blue, greenand red light, then it is of course necessary for both the upper layerand the lower layer to be panchromatically sensitized and the sensitvityof the upper layer must be higher by the same amount than that of thelower layer for each of the said regions of the spectrum.

I claim:

1. A light sensitive photographic material of improved speed andgranularity essentially consisting of a support and in superposedrelationship a first silver halide emulsion of silver bromide or silverbromo-iodide closer to the support and a second silver halide emulsionof silver bromide or silver bromo-iodide superposed over the firstemulsion wherein the improvement comprises the second emulsion is in alayer having a thickness of at least 2 1. and the first emulsion is in alayer having a thickness of between 5 and 15a and is greater inthickness, the first emulsion has a sensitivity about 0.1 to 0.4 log I.tunits less than the sensitivity of the second emulsion as measured bythe points on the characteristic curves which are above the exposurerequired to produce visible density, and which improvement results inthe first emulsion giving a density of at least 1.5, the second emulsiongiving a density of not over 0.9 and the slower emulsion having thethreshold of its characteristic curve coinciding in log 1.1 units withthe shoulder of the characteristic curve of the faster emulsion andwhereby the overall sensitivity in log I.t units of said light sensitivematerial is greater than the sensitivity of the more sensitive secondemulsion measured by the points on the characteristic curve above theexposure required to produce visible density.

2. The photographic material claimed in claim 1, wherein the moresensitive silver halide emulsion has a density between 0.5 and 0.7.

3. The photographic material claimed in claim 1, wherein both silverhalide emulsion layers are panchromatically sensitized and thesensitivities of the two layers for blue, green and red light aredifferent by a ratio in the range of 0.1 to 0.4 log 1.1 units which isthe ratio of the sensitivity of the lower emulsion to the sensitivity ofthe upper emulsion.

References Cited UNITED STATES PATENTS 2,996,382 8/1961 Luckey 96683,050,391 8/1962 Thompson 9668 3,140,179 7/1964 Russell 9668 3,141,7737/1964 Hellmig 96-68 3,353,957 11/1967 Blake 9668 3,402,046 9/ 1968Zwick 9674 3,418,123 12/1968 Haugh 9668 3,450,536 6/1969 Wyckoff 96683,505,068 4/1970 Beckett 9668 3,591,382 7/1971 Millikan 9668 RONALD H.SMITH, Primary Examiner A. T. S. PIC-O, Assistant Examiner US. Cl. X.R.96107

1. A LIGHT SENSITIVE PHOTOGRAPHIC MATERIAL OF IMPROVED SPEED ANDGRANURALITY ESSENTIALLY CONSISTING OF A SUPPORT AND IN SUPERPOSEDRELATIONSHIP A FIRST SILVER HALIDE EMULSION OF SILVER BROMIDE OR SILVERBROMO-IODIDE CLOSER TO THE SUPPORT AND A SECOND SILVER HALIDE EMULSIONOF SILVER BROMIDE OR SILVER BROMO-IODIDE SUPERPOSED OVER THE FIRSTEMULSION WHEREIN THE IMPROVEMENT COMPRISES THE SECOND EMULSION IS IN ALAYER HAVING A THICKNESS OF AT LAST 2$ AND THE FIRST EMULSION IS IN ALAYER HAVING A THICKNESS OF BETWEEN 5 AND 15U AND IS GREATER INTHICKNESS, THE FIRST EMULSION HAS A SENSITIVITY ABOUT 0.1 TO 0.4 LOG 1.TUNITS LESS THAN THE SENSITIVITY OF THE SECOND EMULSION AS MEASURED BYTHE POINTS ON THE CHARACTERISTIC CURVES WHICH